1. Field of the Invention
The present invention relates to load control devices of lighting control systems for controlling the amount of power delivered to an electrical load from a power distribution system. More particularly, the present invention relates to a method of controlling a load control module of a lighting control system, such that the lighting control system is operable to power up the plurality of load control devices in a sequence to reduce stress on the power distribution system at an initial power up.
2. Description of the Related Art
Power distribution systems are often susceptible to abnormal operation in response to the current drawn from the loads connected to the power distribution system. For example, if all of the loads connected to the power distribution system power up concurrently and draw a large electrical current from the power distribution system, the magnitude and frequency of the output voltage of the power distribution system may fluctuate causing undesired responses in the operation of the loads.
The abnormal operation of a power distribution system is commonly brought about by two characteristics of the power distribution system. First, the power distribution system may have a limited peak power capability. If the power distribution system is subject to a pulse of load current having a magnitude that exceeds the peak power capability, fluctuations may occur in the output voltage of the power distribution system. For example, site supply generators have a substantially limited peak power capability as compared to utility-based generation. However, site supply generators are often used as the power distribution systems on marine vessels, such as yachts and cruise ships, and as backup power sources (i.e., in the case of a utility power outage).
Further, power distribution systems having a high source impedance are more susceptible to abnormal output performance. For example, if a residence (i.e., a utilization point) is located a long distance from an electricity generating plant (i.e., a generation point), there is typically a large impedance between the utilization point and the generation point because of the large resistance of the electrical wire between the residence and the generating plant. Accordingly, the output voltage provided to the residence by the power distribution system is more susceptible to fluctuations in the line voltage in response to changes in the load current. The type and size of transformers and conductors used in the power distribution system (such as a generator) may also contribute to a high source impedance.
A typical load of a power distribution system is a lighting control system, which may comprise a large number of lighting loads that are controlled from, for example, a plurality of load control modules located in power panels. The lighting control system may also comprise a central processor for control of the load control modules. Prior art lighting control systems have operated to turn the lighting loads on at once upon power up, i.e., when the lighting control system is energized. Typically, the lighting loads are turned on to the last lighting intensity, i.e., the lighting intensity that the lighting load was illuminated to before the power was removed from the system. A typical lighting control system is described in greater detail in U.S. Pat. No. 6,803,728, issued Oct. 12, 2004, entitled SYSTEM FOR CONTROL OF DEVICES, the entire disclosure of which is hereby incorporated by reference.
When a lighting load is first turned on, the lighting load may draw a substantially large inrush current. Accordingly, if the power distribution system powering the lighting control system is susceptible to abnormal operation as described above, the power distribution system may not be able to provide the appropriate power to start up the lighting control system when the lighting control system is energized such that all of the lighting loads turn on at once. This may occur, for example, when a backup generator powers up in response to a power outage.
Further, a situation may occur in which the output voltage of the generator fluctuates as the lighting control system and all other loads powered by the generator attempts to power up at once. When the generator first powers up, the generator produces an output voltage having a maximum magnitude. After being energized by the output voltage of the generator, the central processor of the lighting control system turns on the lighting loads. The lighting control system may then draw a substantially large inrush current from the generator. If the generator is not able to provide the amount of current required by the large inrush current, the output voltage of the generator decreases in magnitude. If the output voltage of the generator drops to a magnitude that is too low to power the lighting control system (i.e., a magnitude at which the internal power supplies of the components of the lighting control system drop out), the lighting control system turns all of the lighting loads off and stops drawing a significant amount of current from the power distribution system. Since the generator is no longer overloaded, the output voltage of the generator increases in magnitude. Accordingly, the lighting control system powers up, thus, turning all of the lighting loads on again, and the cycle repeats.
Therefore, there is a need for a lighting control system that is operable to start up without over-stressing a power distribution system with a limited peak power capability or a high source impedance.